This is kinda of amazing actually. Could you imagine someone like MS starting Windows over every 7 years so they can be using the latest and greatest coding techniques? I'm guessing they would loose a lot of backward compatibility, but maybe not.
Don't they? NT 3 to NT 4 was only about three years. NT 4 to NT 5 was four years. NT 5 to NT 6 was seven years. Do they completely rewrite? We don't actually know. But they heavily rewrite. It's a major overhaul.
If they lost backward compatibility, that would only be from products relying on broken parts of the old system, not the rewrite. Properly written software running on top would not be affected.
Does Linux get rewritten like this? No. But huge portions of it are continuously refactored in this way or just left untouched.